HelWin1 offshore platform

Siemens has installed the HelWin1 offshore platform in the North Sea, marking the successful completion of a significant stage in the German grid connection project. HelWin1 will link the two offshore wind farms Nordsee Ost and Meerwind to the mainland.

The platform will convert up to 576 megawatts (MW) of electricity produced by the wind farms to direct-current and transmit it via submarine cable to the German mainland 85 kilometers away. The low-loss direct-current power is then converted back into alternating-current at a converter station on shore and fed into the German power grid. Thanks to Siemens direct-current technology, transmission losses are less than four percent.

Location

After seven days of transport at sea and four days of installation on site, the HelWin1 platform was anchored at its final position northwest of the island of Helgoland on August 23. Siemens had installed the platform's support substructure at this location in June in the 23-meter-deep sea waters. The HelWin1 offshore platform solution is designed for decades of operation in the rugged North Sea.

Videos: Stages of construction

Construction

The platform is equipped with a helipad and was fabricated by Nordic Yards at the shipyard in Wismar under contract by Siemens. At 12,000 tons, the HelWin1 platform weighs more than 20 loaded and fully tanked Airbus A380 super airliners. The platform's seven decks accommodate 16 cabins for crew members with a total of 24 berths, a galley, sanitary facilities, a multi-purpose room equipped with sports equipment, and a day room with satellite TV, in addition to housing all the technology and equipment required for HVDC transmission.

TransformersThe video shows skilled workers fabricating and assembling the components for the transformers. They perform careful testing before the enormous unit is lowered into its housing and work on the module is completed. Huge cranes are shown loading the transformer module onto a truck, and it is transported to the harbor where it is loaded onto a barge and shipped to the wind farm location. We see workers laying the subsea cable connections at the onshore substation. Finally, the transformer module is shipped out to HelWin1 and installed in the heart of the platform.

ConvertersUsing the Siemens technology installed on the platform, the alternating-current power generated by the wind turbines is transformed into low-loss direct current for transmission onto land. Starting in 2014, this will enable the network operator and purchaser TenneT to supply clean electricity to more than 500,000 German households on the mainland

Cable layingThe two subsea cables from the well-established cable specialist Prysmian are being connected to privide grid access. Siemens' consortium partner has supplied, bundled and laid the two cables, each with a length of 130 kilometers, for transmission of 250-kilovolt DC voltage in the future. The cable route covers 85 kilometers at sea and 45 kilometers over land. Each 11 centimeter-thick cable, with a protective-steel armoring wire layer and a 35 millimeter-thick copper conductor, weighs about 30 kilograms per meter.

Offshore preparationThe HelWin1 offshore platform is installed on a supporting substructure. It left the shipyard in June and was anchored in the North Sea, which is 23 meters deep at this location, using ten steel piles. The steel pilings - up to 3.2 meters in diameter and with a wall thickness of eight centimeters - were anchored in the seabed to support the structure. Up to 100 meters long, they are as tall as the famous London landmark Big Ben and provide secure anchoring for the platform. The platform is installed 22 meters above sea level to protect it against giant waves.

Going offshoreThree tugboats were needed to tow the converter platform, weighing 11,000 tons, to its location off the coast of Helgoland. Because this steel giant was too large for passage through the North Sea/Baltic Sea (Kiel) canal, it had to be towed around the northern tip of Denmark. After seven days at sea, covering 990 kilometers, HelWin1 finally reached its installation destination.

How does it work?

Thanks to the efficient direct-current technology employed, the total transmission losses for the HelWin1 offshore platform solution connection are less than four percent. The HVDC PLUS technology employed by Siemens reduces the complexity and therefore the space requirements for HVDC systems, which is a decisive factor for installation at sea. In contrast to classic HVDC technology used in a vast majority of land links, systems equipped with HVDC PLUS feature self-stabilization. This enhances grid reliability in the event of power fluctuations, which can occur with wind-based power generation, and thus makes grid access possible for more renewable sources. HVDC PLUS is a multi-level VSC system. This modular, multi-level VSC technology ensures an almost ideal sinusoidal AC waveform and smooth direct voltage along the transmission line, which makes the installation of high-frequency and harmonic filters practically superfluous.